Network design options include contention-free networks and contention-based networks. In a typical contention-free network, one of the devices on the network manages time slot use. In response to requests from other devices wanting to send a data stream, available time slots are allocated by a time slot manager. Once assigned, a transmitter can generally continue to use a time slot, uninterrupted by other streams, until the transmission ends. The original data stream packet intervals that are transmitted over the network are reconstructed by a receiver on the network. An advantage of contention-free communication is that quality of service (QoS) is very high.
However, contention-free networks also have some disadvantages. For example, if the time slot manager is disconnected, another device must take over the time slot management, thus requiring that other devices on the network also be configured with time slot management capability which, in turn, increases network device cost. Furthermore, in response to device connection or disconnection, a full bus reset occurs that disturbs bus communications. Contention-free control operates satisfactorily on a dedicated, noise-free network such as IEEE 1394, but it often is not well suited for use on networks subject to harsh transmission conditions such as an 802.11 wireless network or a power-line communications (PLC) network.
In a typical contention-based network (such as, for example, an Ethernet network), traffic is often controlled using the Carrier Sense Multiple Access (CSMA) protocol. Under CSMA, when one device on the network wants to communicate with another device, the transmitter first detects whether there is a carrier on the bus. If no carrier is detected, then the device commences communication. If however a carrier is detected, then the transmitter enters a back-off mode, and after a delay, attempts the process once again. The wait time during back-off is generally randomized so that two waiting devices do not collide again. Under a conventional CSMA protocol, the bus is not controlled by a time slot manager.
Contention-based networks have both advantages and disadvantages. One advantage is that users are allowed to freely connect or disconnect devices on or from the network without the penalty of a bus reset. Each device need not be concerned with the network status, i.e., what devices are or are not connected to the network. The network interface is simple and low-cost, because no time slot management capability is required. A disadvantage is that QoS is not guaranteed, and therefore when the network is busy, devices are subject to indeterminate wait intervals.
Currently, advances in network design have resulted in an expanded use of home networks. Popular standards include the 802.11 wireless standard and the “HomePlug™ 1.0” power line communications (PLC) network standard. Both of these standards are Ethernet-equivalent and do not provide QoS for audio-visual (AV) data streaming or other uses. Although other network standards such as IEEE 1394 provide QoS, they are not well suited to a number of applications due to a limited cable transmission range, and because cables must be routed from device to device, such as between rooms in a household.
While the existing PLC standard does not have QoS, a next generation PLC standard, “HomePlug™ AV,” is expected to have this. Under this standard, beacon cycles are divided into a contention period (CP) that employs a CSMA or similar protocol and a contention-free period (CFP). Contention-based communications will use the CP which is on first-come, first-served basis. No time allocation is reserved within this portion of the beacon cycle. The CFP is used for contention-free data transmission, especially AV streaming and voice over Internet protocol (VoIP) streaming. Once a time allocation is reserved within the CFP portion of the beacon cycle, the transmitter uses that allocation period, or “channel,” every beacon cycle until all of the data is sent and the transmission terminates. No other traffic is permitted to disturb the transmission.
PLC networks operate under harsh conditions. Signal attenuation is high—especially for a long distance transmission. Also, there are many noise sources, such as for example, hair dryers, lamp dimmers, switching regulator power supplies, etc. Thus packet errors do occur occasionally in these conditions. To solve these errors, a transmitter in a PLC network needs quality of service (QoS) transmission capabilities, i.e., to retransmit the erroneous or missed packets. Usually, a contention-free transmission (such as, for example, the transmission of AV data) reserves only a required minimum bandwidth in the CFP. There is no bandwidth reserved for retransmission of missing packets.
Reserving a bandwidth or channel for QoS transmissions would provide a useful feature for the transmission of AV data or other data over a PLC network. However reserving a channel for QoS transmissions represents an inefficiency in the use of overall CFP bandwidth, because such a channel would not be used for significant amounts of time. Thus there is a need to improve the use of bandwidth while at the same time allowing bandwidth for error recovery or other priority uses.